Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device including a ceramic circuit substrate.
Description of the Background Art
Conventionally, a ceramic circuit substrate has been used in a power semiconductor device such as a GTO (Gate Turn Off Thyristor) and an IGBT (Insulated Gate Bipolar Transistor). In this ceramic circuit substrate, a ceramic substrate has one surface to which a metal circuit board is joined and the other surface to which a metal heat-dissipation plate is joined. Then, a semiconductor element is mounted on the metal circuit board, and a heat sink is joined to the metal heat-dissipation plate.
Since the metal circuit board is greater in coefficient of thermal expansion than the ceramic substrate, the metal circuit board expands more than the ceramic substrate due to the heat produced when using the semiconductor element. Accordingly, warpage occurs in the ceramic substrate.
For example, Japanese Patent Laying-Open No. 10-200219 discloses a circuit substrate including a ceramic substrate having: one surface on which a metal circuit board is formed; and the opposite surface on which a metal heat-dissipation plate is formed. According to the circuit substrate disclosed in this publication, the ratio between the thickness of the metal circuit board and the thickness of the metal heat-dissipation plate as well as the margin width between the outer circumferential edge of the ceramic substrate and the outer circumferential edge of each of the metal circuit and the metal heat-dissipation plate are set so as to fall within a prescribed numerical value range. Thereby, occurrence of cracking in the ceramic substrate is suppressed.
In the circuit substrate disclosed in the above-mentioned publication, the circuit pattern of the metal circuit board is not taken into consideration. When the metal circuit board is simply increased in thickness without any consideration of the circuit pattern of the metal circuit board, the expansion amount of the metal circuit board is increased due to the heat generated when using the semiconductor element. Accordingly, large warpage occurs in the ceramic substrate, which may lead to breakage of the ceramic substrate.